Why upgrade Raspberry Pi to Debian/Raspbian Stretch 9

It’s worthwhile to upgrade to Debian Stretch 9, to go from system Python 3.4 to 3.5, get QT5, etc.
I found it was not worthwhile to upgrade Raspbian in place, as it took several hours and filled my 8 GB SD card.
The best way to upgrade the Raspberry Pi operating system is to wipe (erase) the SD card and reload with Debian/Raspbian from scratch as described below.

copy any wanted files from Raspberry Pi ~ home directory to your PC with sftp or rsync.
The contents of your SD card are erased in the following procedure.

Download Raspbian Image

wget https://downloads.raspberrypi.org/raspbian_lite_latest

Write Raspbian image to SD card: before and after inserting the SD card into your PC, type